The growth hormone secreting somatotrophs of the anterior pituitary are being used as a model for studying plasticity in adult nerve and endocrine cells. These cells can be induced to transdifferentiate to prolactin secreting mammotrophs in vivo and in vitro. Preliminary studies identified a serum factor which stimulates the secretion of prolactin from purified rat somatotrophs in vitro. This factor appears to induce a progressive modulation of secretory phenotype from somatotroph to mammotroph via a somatomammotroph intermediate, a pattern similar to the sequence of mammotroph differentiation in vivo. The specific aims of this project are: l) To further purify and characterize the serum factor described above, 2) To determine whether the membrane ion channels which regulate the levels of hormone secreted are also modulated when pituitary cells switch the hormone they secrete during development, or under the influence of growth factors, hormones or serum factors; and 3) To analyze correlation between ion channel and the hormonal phenotype in human pituitary cells and in rat cells from animals of different ages, sexes, and physiological state (pregnancy/lactation/transformed cell lines). Somatotrophs will be isolated from rats and fractionated by fluorescence activated cell sorting (FACS). Cells will be cultured in a serum-free defined medium or in media containing differentiation factors. Changes in hormone expression and secretion will be assayed in single cells using immunocytochemistry, reverse hemolytic plaque assay(RHPA) and by quantitative RT-PCR. Membrane ion channels will be analyzed by patch clamp techniques. Human pituitary adenoma cells, obtained through the Cooperative Human Tissue Network, will be cultured and assayed for changes in hormone secretion, cell division and ion channel activity.